Occurrence of Pollen and Spores in the Roraima Formation of Venezuela
and British Guiana

The
discovery of pollen and spores in beds considered Precambrian (Proterozoic) has
received brief notice in geological journals and the press[1], [2], [3]. Individual authors will doubtless
publish detailed stratigraphic and palynologic accounts of the occurrence in
due course. Meanwhile it is considered desirable to give an outline of the
facts of the case before distorted interpretations develop from inadequate
data. The following summary statement has been prepared jointly by several
members of the Asociación Venezolana de Geología, Minería y Petróleo. A. single
author is nominated to simplify bibliographic references.

Late in
1963 U. C. K. Dunsterville made an expedition to collect orchids around Cerro
Venamo, at the westernmost point on the frontier between Venezuela and British
Guiana (where this mountain is known as Wenamu Head). He noted some shale‑like
beds at the base of a towering cliff of Roraima sandstone and collected samples
for their possible paleontological interest.

This
discovery of pollen and spores in a formation of supposed Precambrian age was
so remarkable that a reconnaissance expedition of qualified geologists was
organized to verify the facts of the case. During April, 1964, with the
assistance of personnel and vehicles of the Ministerio de Minas e
Hidrocarburos, the locality was visited by a party which included N. Benain, P.
J. Bermúdez, A. Espejo, U. Fournier, A. Menéndez, J. A. Sulek and F. Wright.
They confirmed the salient facts as recorded by Dunsterville. The shale‑like
beds, being less competent, had eroded away below the massive Roraima
sandstone, leaving an undercut extending 10‑12 ft. inwards at the base of the
cliff. The original samples were loose, weathered fragments from the talus
slope below. New samples of unweathered rock were collected from the face of
the undercut.

On their return to Caracas, the three palynologists made independent investigations of the new samples. Utmost care was taken to avoid any possibility of superficial contamination. The rock cleaves along finely laminated bedding planes which are coated with limonite. Every effort was made to avoid these planes and some of the pieces processed were the central nubs left after chipping away the external parts of large blocks of the rock, which was dense enough to sound when struck with a hammer. Nevertheless, microfossils of the same type as before were recovered.

Dr. P. H.
A. Martin‑Kaye, director of the British Guiana Geological Survey, was told of
these discoveries in view of his group's sponsorship of radiometric dating of
the Roraima Formation. He collaborated by sending samples of beds described as
‘hornfelsized sediments’ from a seemingly correlative level near Paruima on the
Kamarang River, some 25 miles east of the original locality. As before,
independent investigations were made of this material, and the same flora was
recorded. Still more samples were submitted by the Survey, this time from the
Kopinang area, about 110 miles south‑east of Paruima. In this case, however,
only indeterminate plant microfossils were obtained, not the distinctive suite
recorded at Cerro Venamo and Paruima.

Fig. 2. Paruima hornfels (crossed nicols);
large crystals are all cordierite in micaceous ground, with original fine
quartz banding still preserved; biotite may be seen to right of centre of
field. (x 150)

It needs to
be stated that petrological investigation of the rocks which yielded pollen and
spores showed them to be highly metamorphosed, even though in hand specimens
they somewhat resembled indurated shales (Fig.
1). J. M. Bowen (Cia. Shell do Venezuela) described the samples from both Cerro Venamo and Paruima as true hornfelses in the cordierite-andalusite range, based on thin-section investigations (Fig. 2). Prof. H. H. Hess (Princeton University) confirmed this on the basis of X-ray diffraction investigations, and described the rock as a fairly typical hornfels, largely muscovite plus a little quartz, clay minerals apparently absent, and no chlorite noted. The samples from Paruima were rather more highly metamorphosed, as shown by the presence of biotite.

Fig. 3 shows a highly generalized
regional cross‑section based on the current geological map of British Guiana.
The remarkably horizontal attitude of the Roraima Formation and the persistence
of dolerite sills[4] over vast distances appear to
justify the long‑range correlations indicated. Relative positions are shown of
the polliniferous samples and of samples dated Precambrian by radiometric
techniques.

The palynological
assemblage has not yet been matched conclusively against any known suite, a
fact which is not surprising in view of the distance from control sections of
known age and the highly endemic character of the flora of to‑day, and probably
the geological past, on the isolated Roraima Plateau. G. Fournier, by utilizing
the herbarium of J. Steyermark, was able to compare the recent pollen of the
area with the assemblage in the rocks, and has stated that they are not the
same. The first opinion of Fournier was that the pollen showed Cretaceous
affinities, but closer study has led to preference for a Tertiary age. L.
Nijssen regards the best assemblage, from a Paruima sample, as not older than
Eocene and quite possibly post‑Eocene. J. W. Funkhouser (Bogotá) considers the
pollen indigenous to the samples he processed but claims an age no older than
Miocene, and probably younger; he notes similarities to the flora of the Mesa
Formation (Pliocene‑ ?Pleistocene) of Colombia, presence of pollen of the
Compositae, and an uncompressed preservation highly unusual except in young
sediments. T. van der Hammen (Leiden) recognizes a mixture of Mesozoic and
Cenozoic elements, but suspects that they represent foreign material
concentrated along cleavage planes as, after cleaning fragments ultrasonically,
he found the matrix practically barren.

As to
interpretation of the significance of the fossil pollen and spores, two sharply
divided opinions have been expressed. The writers make no attempt to
adjudicate, but state the two concepts impartially.

One group
adopts the attitude that the radiometric dating[5],[6],[7] of dolerites and a hornfels6 within the Roraima Formation as Precambrian is
beyond dispute, hence the pollen (and spores) must have entered as secondary
contamination. The improbability that pollen could withstand the baking
process, which converted shale to hornfels,

is adduced
as further evidence that the pollen must be allochthonous. The absence of
macroscopic plant remains in the Roraima Formation is also noted, despite its
assumed continental (?fluviatile) origin. It is admitted that entry of the
pollen into its present site defies simple explanation, though some form of
washing in by meteoric waters in the geological past via joints in the
overlying sandstone seems the most probable cause.

The second
group holds that by no conceivable physical means could the pollen (and spores)
have entered the metamorphosed sediments from the outside. They are dense
impermeable rocks compressed by an overburden of hundreds of feet of the
overlying Roraima sandstones. The undercutting at Cerro Venamo suggests that
the cliff has been steadily retreating, hence the face which was sampled must
have been deep within the formation until quite recent times. The Roraima
sandstones are quartzitic, of low permeability, hence carriage of extraneous
pollen through them by percolating water seems highly improbable. Even if this
process could occur, entry of such pollen and spores into the nonporous
hornfels lacks an explanation. Furthermore, if plausibility of this process be
granted, it would have been operative for a long period, and a mixed suite of
spores and pollen should be expected.

In counter‑argument
against the first group, it is claimed that the assertion that pollen and
spores cannot withstand anaerobic baking of their parent shales has never been
tested experimentally. As regards the radiometric dating, there is a
disquieting overlap between stated ages of the Roraima Formation
[5]
,
[6]
and the underlying basement rocks
[7]
, [8], [9]. The latter suffered complex
deformation and vulcanism, and were then deeply peneplaned before being covered
by thousands of feet of Roraima sandstones, and only after these prolonged
events were the dolerites intruded, on which age‑determination of the Roraima
has been based. If the radiometric technique is valid there should be a long
and clear‑cut time‑gap between ages assigned to the basement rocks and to the
Roraima beds. Such a gap does not exist in the experimental results published,
but this discrepancy is glossed over in the latest summary of radiometric
dating in British Guiana7.

As stated,
we offer no solution to the paradox. It is clear, however, that botanist
Dunsterville in his hunt for rare orchids stumbled on a highly intriguing
geological problem.